With the advent of instrument miniaturization, the precision demanded of molded component parts continues to increase. To manufacture a small molded part produced to strict tolerances, (e.g. lenses used for small instruments such as arthroscopes, etc.) both the molding process and the separation of the part from the molded runner must be performed precisely. The process of separating parts from runners, known as degating, often introduces problems into the manufacturing process which rival the actual molding process in complexity. The problems are exacerbated when the parts are small in comparison to the size of the runner and gate system used.
Precise mechanical removal of molded parts from a runner, first requires the runner system and associated parts to be removed from the molding presses and cooled. Once cool, the part is desirably severed from the runner at the theoretical point where the mold gate connects to the component. The theoretical point is called the mold gate/part interface.
However, differences in runner cooling tend to cause random warpage of the runners to varying degrees. Since the runners are not uniform, it is difficult to set automated degating equipment without risking damage to the parts during degating. Attempts have been made to make warp free runners that are more uniform. However, it was found that runner warpage cannot be reduced without risking a commensurate increase to the part of material strain and stress.
Therefore, it is difficult to achieve a desirable uniformity in automated molding processes such that the parts severed from the runner are in finished form and nearly identical (within allowable tolerances). If the cut is made too close to the part, surface damage may occur on the piece. If the cut is too far from the part, excess gate material will be left still attached to the part. Further, it is difficult to locate the knives or other cutting surfaces to contact the area of the gate closest to the part during the cutting process. Still further, opposing knife edges may not meet precisely in point-to-point contact which results in damage to the cutting surfaces, or at least premature blade wear. Under such circumstances, a gate scar will result on the part which requires an additional trimming or polishing operation to achieve a precision cut tangent to the surface of the part. Processing accuracy is often important in many applications, but especially so in the field of precision optics where lens deviations as small as 1/40 mm may adversely impact the part's usefulness and lead to waste and increased manufacturing cost.
Additional problems occur in degating parts from a runner system when the cutting surfaces are heated to assist in cutting thermal sensitive materials, such as plastics. Often the knives are heated, and the gate/part interface contacted by the heated cutting surface softens to a significant degree. If the severed part and the gate are left in close proximity to each other following cutting, the runner and severed part may readhere to one another.
Still further, as the runner or part (after cutting) is pulled away from a heated cutting surface, even an extremely clean cut may result in "stringers", or string-like tendrils of plastic being formed. Such stringers must then be trimmed from the part and cleaned off from the degating apparatus.